I use all three on a daily basis... My main office machine is Vista, my product and laptop are Linux and my home machine is an iMac, so I'm pretty well versed in all three.
I'll say this, the Vista machine is not as bad as I thought it would be.. the Mac hardware and operating system is a fantastic combination of engineering and HMI design, Linux is an incredible operating system for free, give it 3/4 years more development with continued support from the likes of Dell and I think it'll surpass both Vista and OSX on all levels.
In terms of cost.. the iMac is the only one I've paid for with my own cash, it's the best way for me run and have all three major operating systems available to run natively... and by god does it look good!
'95 Kona Kilauea
'08 Rock Lobster Team Tig SL